U.S. patent application number 10/527721 was filed with the patent office on 2006-06-29 for press-hardened part and method for the production thereof.
Invention is credited to Martin Brodt, Uwe Fischer, Ralf Mehrholz.
Application Number | 20060137779 10/527721 |
Document ID | / |
Family ID | 32094615 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060137779 |
Kind Code |
A1 |
Brodt; Martin ; et
al. |
June 29, 2006 |
Press-hardened part and method for the production thereof
Abstract
To produce a metallic shaped part (1), in particular a vehicle
body part, from a semifinished product (2) made of an unhardened
hot-workable steel sheet, first of all the semifinished product (2)
is formed by a cold-forming method, in particular a drawing method,
into a part blank (10) (process step II). The part blank (10) is
then trimmed at the margins to a marginal contour (12')
approximately corresponding to the part (1) to be produced (process
step III). Finally, the trimmed part blank (17) is heated and
press-hardened in a hot-forming tool (23) (process step IV). The
part (1) produced in the process already has the desired marginal
contour (24) after the hot forming, so that the final trimming of
the part margin is dispensed with. In this way, the cycle times
during the production of hardened parts of steel sheet can be
considerably reduced.
Inventors: |
Brodt; Martin; (Weil der
Stadt, DE) ; Fischer; Uwe; (Eutingen, DE) ;
Mehrholz; Ralf; (Stuttgart, DE) |
Correspondence
Address: |
AKERMAN SENTERFITT
P.O. BOX 3188
WEST PALM BEACH
FL
33402-3188
US
|
Family ID: |
32094615 |
Appl. No.: |
10/527721 |
Filed: |
August 29, 2003 |
PCT Filed: |
August 29, 2003 |
PCT NO: |
PCT/EP03/09607 |
371 Date: |
October 20, 2005 |
Current U.S.
Class: |
148/567 ;
148/650 |
Current CPC
Class: |
C21D 7/13 20130101; C21D
1/673 20130101; B21D 53/88 20130101; B21D 35/00 20130101 |
Class at
Publication: |
148/567 ;
148/650 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 2002 |
DE |
102 42 709.7 |
Nov 23, 2002 |
DE |
102 54 695.9 |
Claims
1. A method of producing a metallic shaped part from a semifinished
product made of an unhardened hot-workable steel sheet, comprising
the following method steps: (I)--providing a semifinished product;
(II)--forming a part blank (10) from the semifinished product (2)
by a cold-forming method; (III)--trimming the part blank (10) at
the margins to a marginal contour (12') approximately corresponding
to the part (1) to be produced; (IV)--heating and press-hardening
the trimmed part blank (17) in a hot-forming tool (23).
2. The method as claimed in claim 1, wherein a deep-drawing method
is used for shaping the part blank (10) from the semifinished
product (2).
3. The method as claimed in claim 1, wherein the part blank (10) is
trimmed by a mechanical cutting method (15).
4. The method as claimed in claim 3, wherein the trimming of the
part blank (10) is effected as part of the cold forming.
5. The method as claimed in claim 1, wherein the tool (23) is
cooled with brine.
6. The method as claimed in claim 1, wherein the semifinished
product (2) is made of an air-hardened steel alloy.
7. The method as claimed in claim 1, wherein the heating and hot
forming of the trimmed part blank (17) are effected in an inert-gas
atmosphere (26).
8. The method as claimed in claim 7, wherein (IV)--the part (1) is
cooled after the hot forming down to a temperature below the
martensite temperature, and is provided immediately afterward with
a surface coating, in particular an anti-corrosion coating.
9. The method as claimed in claim 1, wherein the heating of the
trimmed part blank (17) in process step (IV) is effected in a
continuous furnace (21).
10. The method as claimed in claim 1, wherein the heating of the
trimmed part blank (17) in process step (IV) is effected
inductively.
11. A method according to claim 1, wherein said metallic shaped
part is a motor vehicle body part.
12. A method according to claim 1, of producing a metallic shaped
part, wherein said cold-forming method is a drawing method.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a method of producing a metallic
shaped part, in particular a vehicle body part, from a semifinished
product made of an unhardened hot-workable steel sheet.
[0003] 2. Related Art of the Invention
[0004] Many parts, in particular body parts in vehicle
construction, must satisfy stringent requirements with regard to
rigidity and strength. At the same time, in the interests of weight
reduction, the parts are to have as small a material thickness as
possible. In order to meet these two requirements, high-strength
and super-high-strength steel materials, which--depending on
composition and heat treatment--have very high strength, are being
increasingly used. The production of vehicle body parts from these
super-high-strength steel sheets is preferably effected in a
hot-forming process, in which--as described, for example, in DE 100
49 660 A1--a sheet blank is heated and then shaped in a special
shaping tool and hardened. In this case, by the process parameters
during the hot forming being suitably selected, the strength and
toughness values of the part can be specifically set.
[0005] To produce such a part by means of hot forming, first of all
a sheet blank is cut out of a coil, this sheet blank is then heated
above the structural transformation temperature of the steel
materials, above which the material structure is in the austenitic
state, is inserted in the heated state into a forming tool and
formed into the desired part shape and is cooled down while
mechanically fixing the desired forming state, tempering or
hardening of the part being effected.
[0006] However, in order to cut a part produced in this way in a
dimensionally accurate manner, a large outlay in terms of equipment
is required: in particular, very high cutting forces are required
for the cold cutting of hardened materials, which leads to rapid
tool wear and high maintenance costs. Furthermore, the cold
trimming of such high-strength parts is problematical, since, for
example, the part edges trimmed in the cold state have more or less
large burrs, a factor which may lead to rapid crack formation in
the part on account of the high notch sensitivity of the
high-strength materials.
[0007] To avoid these difficulties which occur during the
mechanical trimming of the hardened parts, alternative cutting
methods are often used, such as, for example, laser cutting or
water-jet cutting. High-quality trimming of the edge of the parts
can certainly be achieved by means of these methods, but these
cutting methods work comparatively slowly, since the cycle times
here depend directly on the length of the cut edge and on the
tolerances to be maintained. The final trimming process therefore
produces a bottleneck during the production of hot-formed parts,
which limits the number of parts to be produced per unit of time.
The total cycle time of the part production can certainly be
reduced if--depending on the length of the cut edge--a plurality of
laser or water-jet cutting units working in parallel are provided;
however, this involves high additional investment and logistics
outlay and is therefore disadvantageous.
SUMMARY OF THE INVENTION
[0008] The object of the invention is therefore to improve the
method sequence during the production of parts of hot-workable
sheets to the effect that the cycle time--irrespective of the
length of the part outer contour--can be reduced.
[0009] The object is achieved according to the invention by the
features of claim 1.
[0010] The essence of the invention consists in the idea that the
part production process should be configured in such a way that the
costly final trimming, which is complicated in terms of the
process, of the hardened part can be dispensed with. According to
the invention, therefore, the marginal regions are already cut off
in the unhardened state of the part, and not only after the heating
and hardening process, as is conventional practice during the hot
forming.
[0011] The production process according to the invention therefore
makes provision for a sheet blank to first of all be cut out from a
coil of hot-workable steel sheet. A part blank is then formed from
this sheet blank by means of a conventional cold-forming method,
e.g. deep drawing, and subsequent trimming of the marginal regions,
this part blank having both (approximately) the desired
three-dimensional shape and (approximately) the desired outer
contour of the finished part. This part blank is then heated to a
temperature above the forming temperature of the material and is
transferred in the hot state into a hot-forming tool, in which the
part is press-hardened. In this method step, the part blank is
formed to a comparatively small extent and is at the same time
subjected to a specific heat treatment, in the course of which
hardening covering the entire part or local hardening is
effected.
[0012] Since the part blank already has approximately the desired
dimensions at the start of the hot forming, only comparatively
slight adaptation or correction of the part contour is required
during the hot forming. As a result, the part margins are changed
only slightly, so that there is no need for final trimming of the
part margins. Here, "part margins" refer to both outer margins and
inner marginal regions (margins of apertures of the part).
[0013] In contrast to conventional hot-forming methods, the
trimming of excess marginal regions in the production method
according to the invention is therefore effected before the hot
forming; at this moment, the part blank is in a soft (unhardened)
state and can therefore be trimmed by means of conventional
mechanical methods. The conventional laser or water-jet trimming of
the finished pressed part can therefore be dispensed with, so that
the processing times can be considerably reduced compared with the
conventional process sequence. At the same time, a high-quality cut
edge is achieved.
[0014] Furthermore, when using the method according to the
invention, the part is now formed only slightly in the hot-forming
tool; the tool wear of the hot-forming tool can therefore be
considerably reduced.
[0015] Since the part geometry is produced (almost) completely by
cold forming, the production of the part can be validated during
the design phase by conventional forming simulations. This enables
development costs for part and tool to be reduced.
[0016] Particular advantages can be achieved if the cold-forming
method used for shaping the part geometry to near net shape is a
(multistage) deep-drawing method (see claim 2). Since multistage
formability of the part blank is possible in the soft state,
complex part geometries can also be shaped. Cutting tools are
advantageously provided in the last stage of the deep-drawing tool,
so that the trimming of the part blank is effected directly in the
cold-forming tool.
[0017] Mechanical cutting means are preferably used for trimming
the part blank (see claim 3). These cutting means may be integrated
in the cold-forming tool in the form of edging and/or punching
tools, so that the trimming of the margins is not effected in a
separate method step but as part of the cold forming (see claim
4).
[0018] In order to be able to further reduce the cycle time of the
entire process, it is advantageous to design the process step of
the press hardening of the trimmed part blank to be as brief as
possible in order to ensure as high a throughput of parts as
possible per hot-forming tool. To this end, the finish-shaped part
should be cooled down as rapidly as possible. In an advantageous
embodiment, the finish-shaped part is quenched in a tool which is
cooled by means of a brine (at a temperature of <0.degree. C.)
as cooling medium (see claim 5); such a brine has especially high
thermal conductivity and thermal capacity. In this way, especially
rapid cooling of the part can be achieved.
[0019] An additional reduction in the cycle time of the entire
process can be achieved if the part is cooled down over a plurality
of stations (correspondingly a plurality of tool sets). Thus, in a
first station, the part is cooled down until the temperature drops
below the martensite boundary temperature. The part strength is
then already sufficient for further transport to the next station
(or the next tool). In this second station (or a sequence of
further stations), the part is then cooled down to hand
temperature.
[0020] In an advantageous configuration, a semifinished product
made of an air-hardened steel is used for producing the part (see
claim 6). An advantage of air-hardened steels consists in the fact
that, in principle, no additional cooling (e.g. by the hot-forming
tool) is necessary for the quenching of the part. In this case, the
part blank is shaped to net shape in the hot-forming tool and then
cooled in the hot-forming tool only until sufficient thermal
stability, rigidity and associated dimensional accuracy of the part
are achieved. The part can then be removed from the hot-forming
tool and be finally cooled in the air; the hot-forming tool is thus
available for receiving a further part blank. In this way, the
cycle times during the production of hardened parts can be further
reduced. If the air hardening is effected under an inert gas, this
results in the further advantage, in addition to this gain in time,
that no scale forms on the part and thus the complicated subsequent
de-scaling is dispensed with (see claim 7).
[0021] During such heating and heat treatment under inert gas, the
part remains free of surface contaminants and can therefore be
advantageously subjected to a surface coating directly following
the hot forming and quenching (i.e. after cooling down to a
temperature below the martensite temperature) (see claim 8). In the
course of this surface coating, in particular corrosion-inhibiting
protective coatings (e.g. by galvanizing) can be applied to the
surface of the part. In this case, the residual heat originating
from the hot forming and remaining in the part can be directly
utilized. Further heat treatment of the part by tempering can then
be effected.
[0022] The heating of the trimmed part blank before the hot forming
may be effected in a continuous furnace (see claim 9).
Alternatively, the heating is carried out inductively (see claim
10). Such inductive heating is effected very quickly, for which
reason an additional gain in the total process time can be achieved
in this case. Furthermore, on account of the short heating
duration, only negligible scaling of the surfaces of the part
occurs during the heating, for which reason the use of inert gas
can be dispensed with. The inductive heating has special advantages
in those applications in which it is not the entire part but only
selected regions of the part that are to be press-hardened: in this
case, by suitable configuration of the inductors, only the regions
to be hardened are selectively heated and then hardened in the
hot-forming tool, whereas the remaining, unheated regions, although
formed in the hot-forming tool, remain in the original ductility.
Alternatively, or additionally, the induction heating enables the
properties of the part to be set over the sheet thickness ("soft
core--hard outer layer"). In this way, locally variable strength
and rigidity properties can be achieved on the finished part.
[0023] For the inductive heating, a separate heating station--in a
similar manner to the continuous furnace--may be provided between
cutting device and hot-forming tool. In contrast to heating in the
continuous furnace--in which a certain heating distance is
necessary--the inductive heating requires less space, a factor
which leads to cost savings. The shape and arrangement of the
inductors is matched to the shape of the trimmed part blank or the
regions to be heated. As an alternative to the heating in a
separate heating station, the heating may also be effected in the
cutting device (directly after the margin trimming) or in the
hot-forming tool (directly before the hot forming). To this end,
the cutting device or the forming tool is provided with internal
inductors, or the part is heated by means of external,
appropriately shaped inductors which are inserted after the margin
trimming or before the hot forming into the opened cutting device
or the opened hot-forming tool and are positioned there at the
desired point of the part.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The invention is explained in more detail below with
reference to an exemplary embodiment shown in the drawings, in
which:
[0025] FIG. 1 shows a method scheme of the production process
according to the invention for producing a press-hardened part:
[0026] FIG. 1a: cutting the blank to size (step I)
[0027] FIG. 1b: cold forming (step II)
[0028] FIG. 1c: trimming the margins (step III)
[0029] FIG. 1d: hot forming (step IV)
[0030] FIG. 1e: dry cleaning (step V);
[0031] FIG. 2 shows perspective views of selected intermediate
stages during the production of the part:
[0032] FIG. 2a: a semifinished product;
[0033] FIG. 2b: a part blank formed therefrom;
[0034] FIG. 2c: a trimmed part blank;
[0035] FIG. 2d: the finished part.
DETAILED DESCRIPTION OF THE INVENTION
[0036] FIGS. 1a to 1e schematically show the method according to
the invention for producing a three-dimensionally shaped,
press-hardened part 1 from a semifinished product 2. In the present
exemplary embodiment, the semifinished product 2 used is a sheet
blank 3 which is cut out of an unwound sheet coil. Alternatively,
the semifinished product used may be a composite sheet which--as
described, for example, in DE 100 49 660 A1--consists of a base
sheet and at least one reinforcing sheet. Furthermore, the
semifinished product used may be a tailored blank which consists of
a plurality of welded-together sheets of different material
thickness and/or different material constitution. Alternatively,
the semifinished product may be a three-dimensionally shaped
sheet-metal part which is produced by any desired forming method
and which is to be subjected to further forming and a
strength/rigidity increase by means of the method according to the
invention.
[0037] The semifinished product 2 consists of a hot-workable steel.
At this point, the air-hardened steel from Benteler sold under the
trade designation BTR 155 may be cited as an example of such a
material, this steel having the alloy composition listed below, in
which case the contents of the alloy partners to be added in
addition to the base metal are to be understood in percentage by
weight: [0038] carbon: 0.18-0.28% [0039] silicon: 0.7% max. [0040]
manganese: 2.00-4.00% [0041] phosphorous: 0.025% max. [0042]
sulfur: 0.010% max. [0043] chromium: 0.7% max. [0044] molybdenum:
0.55% max. [0045] nickel: 0.6% max. [0046] aluminum:
0.020-0.060%
[0047] In a first process step I, the sheet blank 3--as shown in
FIG. 1a--is cut out of an unwound and straightened section of a
coil 5. At this point, the hot-workable material is in a "soft"
(i.e. unhardened) state, so that the sheet blank 3 can be cut out
without any problems by conventional mechanical cutting means--for
example by means of reciprocating shears 4. In large-scale
production use, the blank 3 is preferably cut to size by means of a
blanking press 6, which ensures automated feeding of the coil 5 and
automatic punching-out and discharge of the cut-out sheet blank 3.
The sheet blank 3 cut out in this way is shown in FIG. 2a in a
schematic perspective view.
[0048] The cut-out sheet blanks 3 are deposited on a stack 7 and
are fed in stacked form to a cold-forming station 8 (see FIG. 1b).
Here, in a second process step II, a part blank 10 is formed from
the sheet blank 3 by means of the cold-forming tool 8--a two-stage
deep-drawing tool 9 in the present example. In order to ensure
high-quality shaping of the part geometry in a controlled manner, a
predetermined, optimized material flow on the sheet blank 3 must be
specifically ensured during the cold-forming process. In order to
achieve this, the sheet blank 3 has marginal regions 11 which
project beyond an outer contour 12 (indicated by broken lines in
FIG. 2a) of the part 1 to be formed. Forces are exerted in these
marginal regions 11 by hold-downs 13 during the drawing process,
and these forces produce a specific material flow on the sheet
blank 3 and give rise to a high-quality drawing result.
[0049] In the course of this cold-forming process (process step
II), the part blank 10 is shaped to near net shape. In this case,
"near net shape" refers to the fact that those portions of the
geometry of the final part 1 which are accompanied by a macroscopic
material flow are completely formed in the part blank 10 after
completion of the cold-forming process. After completion of the
cold-forming process (process step II), only slight adaptations of
shape, which require minimum (local) material flow, are therefore
necessary for producing the three-dimensional shape of the part 1;
the part blank 10 is shown in FIG. 2b.
[0050] Depending on the complexity of the part geometry, the
shaping to near net shape may be effected in a single deep-drawing
step or it may be effected in a plurality of stages--for example in
the two-stage deep-drawing press 9 shown in FIG. 1b.
[0051] Following the cold-forming process, the part blank 10 is
inserted into a cutting device 15 and trimmed there (process step
III, FIG. 1c). Since the material of the part blank 10 at this
moment is still in a "soft", i.e. unhardened, state, this trimming
process may be effected by mechanical cutting means 14 (in
particular with cutting blades, edging and/or punching tools).
[0052] A separate cutting device 15--as shown in FIG. 1c--may be
provided for the trimming operation. Alternatively, the cutting
means 14 may be integrated in the last stage 9' of the deep-drawing
tool 9, so that, in addition to the finish shaping of the part
blank 10, the margin trimming may also be effected in the last
deep-drawing stage 9'.
[0053] A near-net-shape trimmed part blank 17 is therefore produced
from the sheet blank 3 by the cold-forming process and the trimming
process (process steps II and III), this trimmed part blank 17,
with regard to both its three-dimensional shape and its marginal
contour 12', deviating only slightly from the desired part shape.
The cut-off marginal regions 11 are discharged in the cutting
device 15; the part blank 17 (FIG. 2c) is removed from the cutting
device 15 by means of a manipulator 19 and fed to the next process
step.
[0054] In the following process step IV (FIG. 1d), the trimmed part
blank 17 is now subjected to hot forming, in the course of which it
is shaped to the final part shape 1 and hardened. To this end, the
trimmed part blank 17 is inserted by means of a manipulator 20 into
a continuous furnace 21, where it is heated to a temperature which
is above the structural transformation temperature in the
austenitic state; depending on the type of steel, this corresponds
to heating to a temperature of between 700.degree. C. and
1100.degree. C. The atmosphere of the continuous furnace 21 is
advantageously rendered inert by a specific and sufficient addition
of an inert gas in order to prevent scaling of uncoated
intersections 12' of the trimmed blanks 17 or--when using uncoated
sheets--on the entire blank surface. The inert gas used may be, for
example, carbon dioxide and/or nitrogen.
[0055] The heated trimmed part blank 17 is then inserted by means
of a manipulator 22 into a hot-forming tool 23, in which the
three-dimensional form and the marginal contour 12' of the trimmed
part blank 17 are given their final, desired size. Since the
trimmed part blank 17 already has dimensions near net shape, only a
slight adaptation of shape is necessary during the hot forming. In
the hot-forming tool 23, the trimmed blank 17 is finish-shaped and
rapidly cooled, as a result of which a fine-grained martensitic or
bainitic material structure is set. This method step corresponds to
hardening of the part 1 and permits specific setting of the
material strength. Details and various configurations of this
hardening process are described, for example, in DE 100 49 660 A1.
In this case, hardening which covers the entire part 1 may be
effected; alternatively, by a suitable form of the hot-forming tool
(e.g. insulating inserts, air gaps, etc.), selected regions of the
part 1 may be omitted from the hardening, so that the part 1 is
only hardened locally.
[0056] If the desired hardening state of the part 1 has been
reached, the part 1 is removed from the hot-forming tool 23. Due to
the fact that the part blank 10 is trimmed to near net shape
preceding the hot-forming process and on account of the adaptation
of shape of the outer margin 12' in the hot-forming tool 23, the
part 1 already has the desired outer contour 24 after completion of
the hot-forming process, so that no time-consuming trimming of the
part margin is necessary after the hot forming.
[0057] In order to achieve rapid quenching of the part 1 in the
course of the hot forming, the part 1 is quenched in a hot-forming
tool 23 cooled by brine. Such brine has a high thermal conductivity
and thermal capacity . . . flows around . . . Depending on the
added salts, the brine can be cooled down to temperatures well
below the freezing point of water.
[0058] As a rule, the hot forming of the part 1 is accompanied by
scaling of the part surface, so that the part 1 has to be de-scaled
in a further method step (process step V, FIG. 1e) in a
dry-cleaning station 25 (for example by means of shot
blasting).
[0059] The method sequence shown in FIGS. 1a to 1e, with the
trimming of the part blanks 10 to near net shape in the soft state,
considerably reduces the cycle time compared with the conventional
method sequence, in which the finished, hardened part is not
trimmed to the final size until after the hot forming by means of
(laser) cutting. If the method according to the invention is used,
the part 1 already has the desired final outer contour 24 after
completion of the hot forming (process step IV), so that the hard
trimming which formed the bottleneck in the conventional method
sequence is dispensed with.
[0060] In the method sequence according to the invention, the
cooling of the finish-shaped part 1 in the hot-forming tool 23 now
constitutes the bottleneck of the entire method: this is because,
during hardening in the tool 23, the cooling time required overall,
depending on sheet thickness, workpiece size and final temperature,
is about 20 to 40 seconds in a good design of the cooling
integrated in the tool, most of the cases being within a range of
between 25 and 30 seconds. A reduction in the cycle time can be
achieved here by using air-hardened steels as materials for the
parts 1: in this case, the part 1 only needs to be cooled down in
the hot-forming tool 23 until sufficient thermal stability,
rigidity and associated dimensional accuracy of the part 1 are
achieved; the part 1 can then be removed from the tool 23, so that
the further heat-treatment process may be effected in the air
outside the tool 23, and the hot-forming tool 23 is available for
receiving a next part blank 17. In this way, the dwell time of the
part 1 in the hot-forming tool 23 can be reduced to a few (<10)
seconds, which leads to a further reduction in the total cycle
time.
[0061] Additional savings or reductions in the cycle time can be
achieved if not only the heating of the part blanks 17 but also the
hot forming is effected in an inert-gas atmosphere; in this case,
the forming tool 23, as indicated by broken lines in FIG. 1d, is
integrated in the inert-gas atmosphere 26 of the continuous furnace
21. As a result, a scale-free press-hardening process is realized,
so that the subsequent dry cleaning, otherwise required previously,
of the parts 1 (process step V) can be dispensed with.
[0062] As an alternative to the heating of the part blanks 17 in
the continuous furnace 21, the heating may be effected
inductively.
* * * * *